How were opals formed...?

[Starstruck8]

... and all my other questions about opals.

(@yssie, I've started this new thread to avoid derailing the other two.)

@Starstruck8 Piggybacking on a convo in another thread - I read this first website and felt like it generated more questions than it answered -
http://opalsociety.org/opal_cutting.htm

More googling, more questionable results... But all reputable pages seem to reference this article - which I've just started to read but seems like it will (actually) answer a lot of the questions we've been tossing out. Interested in your thoughts if you have time to read it! Starts on Pg 34 - Frozen Opal Fluids and Colloidal Crystal Fire: Gem Opal Deposits in the Heart of Australia
https://www.opal.asn.au/wp-content/uploads/2019/01/InColor-41-Winter-2019-complete-22142.pdf

It's a fascinating but frustrating article. The key worry is right at the beginning, p34: 'Despite there being no consensus on genesis at the moment...'. As best I can tell, there still is no consensus. I'd love to be proven wrong!

The play of colour in precious opal is caused by silica spheres of roughly uniform size, arranged in regular arrays. But how were the spheres formed? How did they get to be so uniform in size? How were they arranged into neat arrays?

I'll leave aside the question of how the spheres were formed. Pecover is advocating for his theory that the sorting and arranging was done by flow under pressure - fluid containing spheres was forced into cavities under pressure. Irregularities in the flow caused the spheres to start piling up in regular arrays. He is arguing against the 'traditional' view - fluid containing spheres seeped slowly into cavities and the spheres settled slowly to the bottom, building regular arrays from the bottom up. There are also other stories...

In favour of slow-but-steady bottom-up, you certainly can form opal that way - it's how synthetic opal is made:
https://www.synthetic-opals.com/synthetic-opal-manufacture.html

In favour of flow-under-pressure, some opals show obvious flow structures. Pecover gives examples. This article, cited by Pecover, gives others:
https://www.mdpi.com/2075-163X/8/1/12/htm
(Check out the SEM pictures - 'regular' comes in degrees!) But what about opals that don't show obvious flow structures?

So who is right? Or both or neither? I haven't been able to find any authoritative-looking source that suggests a consensus.

All this relates only to Australian (sedimentary) opals. The mostly 'volcanic' opals found in other parts of the world are a whole different story...

I'd love to hear from anyone with interesting questions or information about opals.

 

[yssie]

Starstruck8 Thank you for starting this thread!

So here's what I got from that article, taking Pecover's theories as fact, and the one right after it - re. Lightning Ridge -

1. Water extracts silica mineral precursors (Si(OH)3O and Si(OH)2O2) from silica rich compounds in the rock.
2. Now there's a ton of those silica precursors dissolved in water - more and more until the solution becomes saturated with silica-to-be.
3. Those silica precursors form actual silica spheres of different sizes in the water solution.

- But why are different sized spheres formed? ​

- And are the medium size spheres that produce blue and green light return more common than big (red) and small (violet) spheres? ​

So at this point you've got silica spheres suspended in water. And Australia is a hotbed of tectonic activity. And it always has been. So

1. The rock split, creating all sorts of fissures.
2. This silica sphere rich water flooded into the splits.
3. In areas where the water's path was super turbulent (that is - closest to the walls of the fissures) the agitation caused the silica spheres in solution to tangle up with each other, and deform, and ultimately create an disorderly silica-rich shear-thickening water suspension...

- So... The more the turbulence the stronger the shear thickening effect... And the more deformed the silica spheres, and the more disorderly the silica sphere remnant arrangement, and the more not-colourful the poch...?​

- What makes black vs. grey vs. white poch? ​

- Is crystal opal mostly clear/glass poch? ​

4. As fluid flow becomes smoother and smoother (further toward the middle of the fissure, away from the walls) the fluid thickens less and less with the shear of each fluid layer. (For a broader audience - fluid flow through a tube is normally modelled in layers so this part isn't as whackadoo as it might sound at first!).
5. Until the point where - most likely in the middle of the fissure - the silica spheres get the chance to pack together in tight and orderly fashion. Like if you had a shallow tray of marbles piled high and lumpy, and you gently swept (sheared) your hand over the top, and they "settled" into a nice orderly matrix.

- And if the tray had marbles of different sizes the small ones would try to settle between the holes that the bigger marbles make, so it'd be more rare to see a "big area" of only big marbles - is this a reasonable layman's explanation for why red is rare? ​

9. Then $Something happens and the silica sphere matrix (or the disorderly mass of silica sphere remnants) is "cemented" with silica precipitate, which takes the place of water. Here's the article's explanation -

Eventual sudden freezing of all hydrodynamic and rheological activity in response to a termination of the prevailing stress field and a rapid drop in fluid pressure and flow velocity, thereby virtually instantly preserving patterns of opal fluid flow in the veins

10. When less water is replaced with silica precipitate the opal's water content is higher and crazing is more likely as it dries out.

My other questions - besides the italics -
- Is it really plausible for a solution to respond SO differently to shear to create poch vs. this orderly matrix of silica spheres? My other half teaches university fluid dynamics and says "yes... maybe... it depends". LOL.
- Pecover cites himself. A lot. @Starstruck8 you saw this too. Like, a lot. Kinda really erodes credibility. (Erodes. Hah). How much is he trusted as an authority this industry?
- What causes the square vs. hex matrices?

I feel like I still have more non-understanding than understanding TBH. But lemme just drop a couple of the pics that I found really interesting here.

 

[yssie]

Also - can’t lie - I come to CS less frequently than other parts of PS and when I do it’s almost always just to gawk - so I feel like a low key fraud actually posting an opinion in here

 

[Starstruck8]

Also - can’t lie - I come to CS less frequently than other parts of PS and when I do it’s almost always just to gawk - so I feel like a low key fraud actually posting an opinion in here

Nothing fraudulent about your interest in opals. I'd say that your collection entitles you to post as many opinions as you like. A fair few more than my 'collection' entitles me to post.

So many questions! I can start with a few of the easy ones...

What causes the square vs hex matrices?
FCC packing shows squares on some planes and triangles on others. A bit hard to visualize, but true.

Is crystal opal mostly clear/glass poch?
Potch (note spelling) is never clear or glassy, always opaque. (Strictly, a bit translucent – you can shine a strong flashlight through it, if it’s not too thick.) So crystal opal is not potch.

What makes black vs. grey vs. white poch?
I’m just guessing (no reference) that the different shades of potch come from impurities. Perfectly pure potch (nothing but silica) would be white. Impurities that absorb light would make the potch darker.

Added: I came across this: The nature and origin of pigments in black opal from Lightning Ridge, New South Wales, Australia. J. R. Herrmann et al.
https://www.tandfonline.com/doi/abs/10.1080/08120099.2019.1587643
I can access only the abstract. (SUPPORT OPEN ACCESS!) They dissolved the silica from some 'black' potch and looked at what was left. They found black carbon and sulfide minerals. They suggest a microbial origin. Note: They implicitly assume a ‘settling’ model of opal formation rather than Pecover’s ‘flow’ model.

Is it really plausible for a solution to respond SO differently to shear to create poch vs. this orderly matrix of silica spheres? My other half teaches university fluid dynamics and says "yes... maybe... it depends".

I tend to agree. But Pecover's story is more than bit handwavy. Maybe his other papers give more detail. It would be nice to see an actual experiment to show that his model really could work. Note that for the 'settling' model, the making of synthetic opal I mentioned above is just such an experiment.

 

[Starstruck8]

@yssie, I found this recent (23 Nov 2023!) article:
Timing of Opalization at Lightning Ridge, Australia: New Evidence from Opalized Fossils. G. E. Mustoe & E. T. Smith:
https://www.mdpi.com/2075-163X/13/12/1471

You may not be very interested in opalized fossils in themselves. But the authors' aim is to use the fossils throw light on theories of opal formation. They confirm what I suspected - that there is no consensus. They list the main theories (Section 5 - Opal Formation Hypotheses). This and the references are a handy up-to-date overview. Pecover's theory ('Syntectonic Opalization') is included. It's clear from the discussion that it's taken seriously - Pecover might quote himself a lot, but he is not obviously regarded as a crank. I have to say that some of the other theories seem a bit, err... weird.

So here's what I got from that article, taking Pecover's theories as fact, and the one right after it - re. Lightning Ridge -

1. Water extracts silica mineral precursors (Si(OH)3O and Si(OH)2O2) from silica rich compounds in the rock.
2. Now there's a ton of those silica precursors dissolved in water - more and more until the solution becomes saturated with silica-to-be.
3. Those silica precursors form actual silica spheres of different sizes in the water solution.
- But why are different sized spheres formed? - And are the medium size spheres that produce blue and green light return more common than big (red) and small (violet) spheres?

As far as I can see, Pecover's article says nothing about how the spheres form - that's just not the bit he's interested in. No doubt, some of the other theories address this. But I haven't had time to digest them - there's a pretty heavy mix of geology and colloid chemistry.

 

[yssie]

@yssie, I found this recent (23 Nov 2023!) article:
Timing of Opalization at Lightning Ridge, Australia: New Evidence from Opalized Fossils. G. E. Mustoe & E. T. Smith:
https://www.mdpi.com/2075-163X/13/12/1471

You may not be very interested in opalized fossils in themselves. But the authors' aim is to use the fossils throw light on theories of opal formation. They confirm what I suspected - that there is no consensus. They list the main theories (Section 5 - Opal Formation Hypotheses). This and the references are a handy up-to-date overview. Pecover's theory ('Syntectonic Opalization') is included. It's clear from the discussion that it's taken seriously - Pecover might quote himself a lot, but he is not obviously regarded as a crank. I have to say that some of the other theories seem a bit, err... weird.

This is actually fascinating to me. My other half and I have gone to Theodore Roosevelt National Park a couple of times and the petrified forests are what I'm waiting for on every hike - sometimes it's a huge area full of old tree trunks of all sizes, and you wonder exactly what happened to ravage them, sometimes it's just one or two trees that apparently "got unlucky"...





You can actually see the opalization of the wood into quartz in some places, the landscape glistens And interestingly - although it's absolutely bone dry now - there are always tons of stones in these areas that look water-worn. Even some shells. My husband noticed as well. There was water here once, lots of it!

Anyway. Thanks for sharing, I'll have a read this week!!

As far as I can see, Pecover's article says nothing about how the spheres form - that's just not the bit he's interested in. No doubt, some of the other theories address this. But I haven't had time to digest them - there's a pretty heavy mix of geology and colloid chemistry.

This is from the article immediately following Pecover's - Water in Opal -- What Can It Tell Us?
Starts on Pg 63 (per the PDF pagination).

Added: I came across this: The nature and origin of pigments in black opal from Lightning Ridge, New South Wales, Australia. J. R. Herrmann et al.
https://www.tandfonline.com/doi/abs/10.1080/08120099.2019.1587643
I can access only the abstract. (SUPPORT OPEN ACCESS!) They dissolved the silica from some 'black' potch and looked at what was left. They found black carbon and sulfide minerals. They suggest a microbial origin. Note: They implicitly assume a ‘settling’ model of opal formation rather than Pecover’s ‘flow’ model.

I've got it. Full PDF attached.

 

[Starstruck8]

This is actually fascinating to me. My other half and I have gone to Theodore Roosevelt National Park a couple of times and the petrified forests are what I'm waiting for on every hike - sometimes it's a huge area full of old tree trunks of all sizes,

Yes, petrified wood is fascinating. There's quite a bit of it in SE Qld, but mostly on private land. At a place I visited as a kid, the farmer had piled it up into big heaps, because it was getting in the way of his machinery. I was intrigued by the amazingly preserved structure and the occasional agate/chalcedony bits. But we don't have dramatic petrified standing forests. I will almost certainly never visit the western US, but it's great to look at your pictures.

and you wonder exactly what happened to ravage them, sometimes it's just one or two trees that apparently "got unlucky"...

Weren't they the ones that got lucky, by being preserved? Everything dies, but almost all of it rots. For a tree be petrified with intact structure, the mineralization has to start before the rot sets in. This requires very special circumstances.

I've got it. Full PDF attached.

Thank you. The full article has value beyond the abstract. (1) It's (almost) clear that they were looking at potch nodules, with no precious opal. So, not quite what's normally understood by 'black opal'. (2) 'Settling' under gravity is obvious - the darker potch is 'pooled' at the bottom of the nodules. (3) The reason they suggest bacterial origin is that pyrite (which they found) breaks down in the oxidizing conditions that their top-down weathering theory requires. This need not apply to bottom-up artesian water theories.

This is from the article immediately following Pecover's - Water in Opal -- What Can It Tell Us?
Starts on Pg 63 (per the PDF pagination).

The first page describes the bulk chemistry of silica polymerization. But under what conditions does she silica form uniform spheres rather than an unstructured gel? That's the tricky bit.

This illustration is from R. K. Iler - The Chemistry of Silica, 1979, p174. Iler's book, despite its age, seems to be the 'bible' that everyone who theorizes about opal refers to. I recommend it.


Basically (so to speak), to form large uniform spheres you need alkaline conditions and a continuous supply of silica. Explaining how this comes about is the crux of any theory of opal formation.

There are competing theories and no consensus. So I'm inclined to give up on this for now. Mastering the required background would be 'non-trivial', to put it mildly. And even if I did, wouldn't I in effect be making up my own unconfirmed theory? Best to wait and see, I think.

That said, I've learned a lot and looked at a lot of interesting SEM pictures. Panel (b) from this figure is the most intriguing:

(From Silica Colloid Ordering in a Dynamic Sedimentary Environment, M Liesegang & R Milke:
https://www.mdpi.com/2075-163X/8/1/12/htm)

This shows precious opal that has replaced multiply twinned calcite (see panel (a)). The arrays of spheres are aligned to the cleavage planes of the calcite! Amazing! The authors propose a theory of progressive replacement at the receding surface of the calcite. See also: Silica nanoparticle aggregation in calcite replacement reactions, M. Liesegang et al.
https://www.nature.com/articles/s41598-017-06458-8

Of course, despite what I said about giving up, I am still interested in anything you have to say.

 

[Starstruck8]

What causes the square vs. hex matrices?

That raises an interesting point, @yssie, that’s not explicitly mentioned in most popular accounts.

First, to answer the question directly: The silica spheres are usually said to be arranged in (imperfect) face-centered cubic arrays. Planes parallel to cube faces show a square pattern. Planes parallel to the octahedron faces show a triangular pattern.

But here’s the interesting bit: this also helps to explain the multiple colours of opals.

The ‘triangular’ planes are spaced more widely than the ‘square’ planes, so they will reflect longer wavelengths. Doing the maths, the ‘triangular’ planes are spaced at sqrt(2/3) of the diameter of the spheres, the ‘square’ planes are spaced at sqrt(1/2) of the diameter. So the ‘square’ planes more closely spaced by a factor of sqrt(3/4) ≈ 0.87.

If you illuminate an opal with a flashlight held close to your eye, the colours you see will depend only on the spacing of the reflecting planes. If all the ‘blocks’ have the same sized spheres (as is often assumed), and all have face-centered cubic packing, there can be (at least) two reflected wavelengths, a longer one from the ‘triangular’ planes, and a shorter one from the ‘square’ planes. For example, a plausible pair might be 530nm (green) and 460nm (blue).

Reality is more complicated. The fcc packing is usually imperfect. This permits other systems of planes, some more widely spaced than the 'triangular' planes. Search on ‘stacking faults’ and ‘reciprocal lattice rods’ if you dare.

That’s the theory. Now the practice. ‘In the interests of science’, I knocked a small chip (about 5x7mm) out of an inexpensive rock shop specimen of crystal opal in matrix.


Here are pictures of the chip in a pool of glycerine. (This has refractive index very close to that of opal, so it makes the rough surface of the chip mostly irrelevant – nearly all the refraction happens at the smooth level surface of the glycerine). Lighting from a LED flashlight.


You can see that even this tiny chip has many ‘blocks’. The colours are green and blue.

Here is a composite of pictures through a spectroscope (small pocket spectroscope, jammed in front of a macro lens on a DSLR). Lighting from direct sun. It takes a lot of fiddle to catch a flash. The outer spectra are for calibration, from a compact fluorescent light and skylight.


The flashes are amazingly pure. Very few things other than lasers or uncoated LEDs are as spectrally pure as opal flashes. This is no doubt why opals look so special.

The wavelengths of the three flashes in the lower opal spectrum as about 435nm, 495nm and 525nm. (The lighting was not directly from the viewing angle, but this should shrink all the ‘flash’ wavelengths by about the same factor.) The ratio of the first two is 0.88. This is close to the theoretical ratio of 0.87 for the spacing ‘square’ planes to ‘triangular’ ones. Coincidence? I wouldn’t like to say.

Note the skylight spectra. They are straight from the camera, shot raw, with adjustment only for exposure. The colours aren’t right. As in, they are not what you see if you look through the spectroscope. There’s not enough yellow. They seem to go straight from green into orange. This is an artifact of my camera’s sensitivity curves and standard colour processing. Now I've been alerted to this, I've actually seen it in some vendors opal videos. The moral: since opal flashes are so spectrally pure, you can’t rely on any camera to capture them accurately.

The best popular account I’ve found of opal colours is an old Scientific American article, written by the early researchers: Opals, P.J. Darragh, A.J. Gaskin and J.V. Sanders: Scientific American, April 1976, pp 84-95.

 

[Bron357]

Opals are fascinating. And the reason that red colour is revered is because the spheres need to grow to largest size to refract red under white light.
And just to throw this in, if the formation of opals isn’t “rare enough” there is the harlequin pattern which requires the assembly of spheres in planes to make distinct squares.
A black opal with lots of red in harlequin pattern - take my kidney and first born ha ha.